1. Field of the Invention
This invention relates to coated optical fibres and methods for their production. In particular the invention relates to coated optical fibres which are especially suitable for use in blown fibre systems.
2. Related Art
Optical fibres are now widely used in place of electrical conductors in the communications field. Typically, glass optical fibres have an external diameter in the range 100-150 .mu.m, usually 125 .mu.m. Polymer fibres are normally somewhat larger in diameter. Unlike conventional electrical conductors, optical fibres are generally fragile and easily damaged to the detriment of their performance and lifetime. Consequently, it is important to protect the fibres from damage.
The first step in the protection of optical fibres occurs, at least in the case of glass fibres, immediately after the fibres are drawn and involves the application of one or two layers of synthetic resin coating. This protection, whether one or two layers, is somewhat loosely referred to as the "primary coating", and fibres so coated are sometimes known as "primary coated fibres". Alternatively, and more accurately, the coatings are sometimes referred to as the primary and secondary coatings, and this convention is adopted in this specification. The first coating, which is typically a low-modulus silicone or acrylate polymer is applied to the fibre surface at a point no more than about a meter from the point where the fibre is drawn down. Commonly, the primary coating is UV curable. The primary coating is also known as the buffer layer, since it serves to buffer the fibre from lateral pressure. The reason for applying the primary coating practically as soon as the fibre is formed is that the strength of glass and other small fibres depends critically on the extent to which their surface is free from cracks and microcracks. In order to avoid the formation of microcracks it is important to protect the fibre surface from dust and other causes of abrasion, and to this end the zone between the point of fibre drawing and the point of application of the primary coating is kept short and dust-free. The mechanical properties of primary coating materials are critical to the performance of optical fibres. In particular, the coating should not induce microbends in the fibre and the mechanical properties should be compatible with those of the fibre.
A particularly important consideration is the material's coefficient of thermal expansion (TCE). The different in TCEs between the material of the fibre (normally a silica-based glass, which means a low TCE) and that of the primary coating (normally with a TCE an order of magnitude or more greater) means that at low temperatures fibres may be subject to considerable compressive stress, significantly increasing optical loss. This effect is generally made worse by increasing the primary coating thickness, and of course with reduced temperatures.
The secondary coating is typically a hard and robust material, such as nylon, to protect the primary coating, and hence the fibre, from damage. [Increasingly, acrylates, e.g. urethane acrylate, are being used in place of nylon.] Again, the physical properties of the material are very important in terms of their effect on the optical performance of the fibre, particularly its temperature sensitivity. Particularly now that optical fibres are being more widely deployed, it is important that optical fibres can be packaged to withstand extremes of temperature. In practice, it is insensitivity to low temperatures, e.g. sub-zero centigrade, which is the most difficult to achieve. For network use in continental climates, it is desirable that optical fibres should show no significant excess loss at temperatures as low as -20.degree., -40.degree. or even -60.degree. C. Some relevant aspects of the temperature sensitivity of optical fibres are dealt with in the following papers:
T. A. Lenahan, A. T. & T. Tech. J., V.64, No. 7, 1985, pp 1565-1584, T. Yabuta, N. Yoshizawa and K. Ishihara, Applied Optics, V.22, No. 15, 1983, pp. 2356-2362; and Y. Katasuyama, Y. Mitsunaga, Y. Ishida and K. Ishihara, Applied Optics, V. 19, No. 24, 1980, pp 4200-4205.
Conventionally, the primary or secondary coated fibres, which typically have a diameter of about 250 .mu.m, are made up into cables which provides the required level of mechanical protection for the optical fibres. It is important to protect the optical fibres from strain, consequently it is usual to decouple the optical fibres from the bulk of the cable structure. Typically, this decoupling is effected by locating the optical fibre(s) in a tube or slot in which the fibre is free to move. In addition to decoupling the fibres, it is necessary to ensure that the rest of the cable structure can withstand the loads which will be applied during installation or use of the cable, without imposing excess strain on its optical fibres. Since the level of strain which optical fibres can endure without damage is very low, typically less than 0.2 percent, cable structures need to be very strong. Typically, optical fibre cables are installed in much the same way as copper wire cables, that is they are pulled into place through ducts and conduits using a rope attached to a cable end. Cables experience very high tensile loadings during such installation, and consequently optical fibre cables need very considerable reinforcement to prevent their optical fibres being damaged. These requirements increase the size, weight and cost of optical fibre cables.
As alternative approach to optical fibre installation is described in our European patent EP-B-0108590. In this method the fibres are installed along a previously installed duct using fluid drag of a gaseous medium which passes through the duct in the desired direction of advance. This method, which is known as Blown Fibre or Fibre Blowing uses distributed viscous drag forces to install a fibre unit which is supported on a cushion of air.
Since the duct is installed first, conveniently using traditional cable installation techniques, without any optical fibres and since there is no significant stress imposed on the fibre unit during blowing, it is possible to use very lightweight fibre structures. Indeed, in terms of space-saving and routing flexibility it is desirable if the fibre unit is both small and flexible. Typically, a fibre unit consists of a plurality of conventionally coated optical fibres held together in a lightweight polymer sheath which has a foamed coating. Such multiple-fibre units may also include a ripcord to facilitate the splitting out of the fibres from the unit for termination of the fibres. Examples of multiple-fibre units are described in our European patent EP-B-0157610 and EP-A-0296836. Fibre units can also usefully consist of just a single fibre provided with a suitably bulky and lightweight sheath, as discussed in EP-B-0157610 and EP-A-029836. An example of a single-fibre unit is described in EP-A-0338854 and EP-A-0338855.
It has been found to be desirable, e.g. for good blowing performance, for the coatings in a fibre unit to surround the fibres tightly. As a result of this, the mechanical properties of the fibre unit coatings are as significant to the temperature sensitivity of the optical fibres as the mechanical properties of the primary and secondary coatings. It is no surprise, therefore, to learn that in EP-A-0296836 the fibre unit coatings comprises: an inner strength of a material which is soft and has a low modulus of elasticity, e.g. an acrylate or thermoplastic rubber; an optical intermediate sheath which is hard (greater than 75 D Shore hardness) and has a high modulus of elasticity (greater than 900 N/nm.sup.2), to confer mechanical protection on the soft sheath, and an outer sheath of foamed material. This arrangement is akin to the primary and secondary coatings whose application to individual fibres was described above, with the addition of a foamed layer to reduce the fibre unit density and hence improve blowability. However, while there is some similarity between the requirements made of primary and secondary coatings and those made of what might be regarded as the tertiary and quaternary coatings, particularly when one is only coating a single fibre, there are extra constraints which only apply when one is provided a coating system which has to hold several fibres together. Thus, in a multiple-fibre unit one would expect to use materials having larger elastic moduli and in considerably greater thicknesses. Moreover, when a multiple-fibre unit is bent, the individual fibres will generally each experience different bending forces and will tend to move relative to each other. In addition, the larger diameter of multiple-fibre units means that for a given bend radius the outer surface of the outer coating is exposed to greater tensile and compressive stress than in a single-fibre unit. It is clear therefore that one cannot necessarily expect a coating system which works on a single-fibre unit to work for a multi-fibre unit. A further consideration is that while one might expect stronger coatings to solve the problem of transition from single--to multiple-fibre units, it has to be borne in mind that the optical properties of the optical fibres in a fibre unit are very dependent on the physical properties of the coatings used in it. In particular, and an mentioned previously, the physical properties of optical fibre coatings markedly affect the temperature sensitivity of optical fibres coated therewith. Moreover, the stiffness of a fibre unit markedly affects its blowing performance. If a fibre unit is too stiff, it will not blow--at least in a real-life environment.
Thus, it is by no means clear that a coating system which works for a single-fibre unit will also work for a multiple fibre unit.
In EP-A-0345968 there is described a range of single-fibre units having an external coating which comprises a radiation-cured polymer containing particulate matter. The particulate matter is variously, PTFE particles, hollow glass microspheres, or hollow polymeric microspheres. The particulate matter, which preferably has an average particle size of less than 60 microns, is mixed in with the un-cured liquid polymer. The fibre to be coated, which may already have a tertiary buffer layer, is drawn through a bath containing the polymer/particulate mixture to give an outer coating having a thickness in the range 10 to 70 microns. The coating is then cured using UV radiation.
We have found that the coating systems as described in EP-A-0345968 are not suitable for use in sheathing multiple-fibre units. In particular, we have found that such coatings on multiple-fibre units tend to fail when the unit is bent.
We have found that, particularly with multiple-fibre units such as 4-fibre and 8-fibre units, the coating system described in EP-A-0345968 for single-fibre units wherein particulate matter is mixed in with the outer coating polymer, produces fibre units which are very prone to "fibre breakout". As a fibre unit is progressively bent, and thus experiences a progressively smaller bend radius, a certain bend radius is reached at which irreversible damage to the sheathing occurs allowing the secondary coated fibres to be exposed. This phenomenon is known as fibre-breakout. If the bend radius at which fibre-breakout occurs (the minimum bend radius) at which fibre unit is likely to experience its minimum bend radius during normal handling of the fibre unit, the unit is in practice not useable.